An interinstitutional cooperative study is proposed to determine the effectiveness of the oral administration of acetylsalicylic acid (aspirin) in the reduction or prevention of transient cerebral ischemic attacks and amaurosis fugax. In addition, the effect of the frequency of cerebral and myocardial infarction in long-term follow-up will be evaluated. This will be a controlled clinical trial in which only individuals having the hemispheric type of transient ischemic attack or monocular blindness will be included. Those with primary vertebrobasilar insufficiency only, or with potentially fatal complicating medical conditions, will be excluded. Care will be taken to exclude individuals found to have blood dyscrasia or specific idiosyncrasy to aspirin. Subjects will be allocated to subgroups on the basis of a clinical decision to operate or not to operate. In each of the two subgroups, subjects will be assigned to treatment with aspirin or a placebo on the basis of a random statistical design. This design will enable the investigators to make meaningful comparisons between therapies in each subgroup. Strict surveillance will be maintained of the patient's adherence to the instructions for therapy.